It is demanded that the optical module incorporated in any camera for use in mobile telephones should be thin. In recent years, the imaging device has come to have more and more pixels to increase the image quality. As the size of each pixel decreases, the depth of field decreases. To increase the depth of field, an optical mask is used.
A technique called apodization is known, whereby an optical mask is provided. In apodization, a filter having transmittance high at center and gradually decreasing toward the periphery is arranged, as optical mask, at the input pupil of an optical module to enhance the image contrast. Apodization reduces high-order diffracted light, increasing the contrast of low-frequency waves. On the other hand, the contrast of high-frequency waves is decreased. The optical filer used in apodization is called an apodizer.
FIG. 10 is a sectional view schematically showing an example of an apodizer. This apodizer H is composed of a transparent part H1 and a black part H2, which are combined together.
In the apodizer of this structure, the film-thickness distribution controls the transmittance. Light is inevitably refracted at the interface between the black part and the air layer. Consequently, the resolution will decrease even if the depth of field increases. Further, if the apodizer is made thicker in the direction the light passes through it, the resolution will decrease.
The apodizer may be manufactured by, for example, bonding a plano-concave lens made of light-absorbing glass and a plano-convex lens made of transparent glass, thereby decreasing the optical power to almost zero. The optical characteristics this apodizer exhibits if incorporated in an optical system are known. A method of manufacturing an apodizer is also known, in which a light-absorbing material is vacuum-deposited through a conical mask member, gradually increasing the optical density of the apodizer, from the optical center toward the periphery.
Any image-forming optical system that incorporates such an apodizer is difficult to make small, the filter being thick because a plano-concave lens of light-absorbing glass and a plano-convex lens of transparent glass are bonded together to constitute the apodizer. Further, the transmittance distribution is difficult to control accurately in the apodizer, because the apodizer has been made by vacuum-depositing a light-absorbing material through a conical mask member. Still further, the vacuum deposition must be performed for a long time to achieve the optical density required, inevitably increasing the cost of manufacturing the apodizer.